Filter tube

ABSTRACT

A filter tube, which filter tube comprises a mass of interrelated nonwoven glass fibers, the fibers having a diameter of from about 0.001 to about 10 microns, the fibers bonded at the junctions of the fiber crossovers with a hardened silicone resin bonding agent, the fibers interrelated to form a semirigid mass of desired porosity suitable for use as a filter for gases or liquids.

BACKGROUND OF THE INVENTION

In various processes requiring the filtration of a gas or liquid stream,filters composed of woven or nonwoven fibers are often employed, eitheralone in flat sheet or tubular form or supported by a suitable poroussupport. In the selection of such filters for a particular application,one characteristic relates to the efficiency, flow rate and life of thefilter. These criteria typically depend upon the particular propertiesof the material of which the fiber is composed, but more particularly,are related to the particular diameter of the fibers used. Anotherimportant criteria is an environmental one as to whether the filterswill withstand the particular pressures, temperatures and physicalnature of the gas or liquid material to be filtered.

As to this latter criteria, filters made from organic material are quitesusceptible to pressure, temperature and the chemical nature of the gasor liquid to be filtered. Filters made from glass fibers have beenemployed where the glass fibers are formed into an interlaced mass ofthe glass fibers without any physical coherency or strength; that is,without a binder to retain the glass fibers in a coherent manner.Although such glass fibers have good temperature and chemicalresistance, the physical strength of such fibers is quite low, and suchfibers may only be used in most undemanding applications. The filtersmust, therefore, be treated with a great deal of care by the user inorder that they remain as useful filters.

The lack of strength of such glass fibers has been overcome by bondingthe glass fibers with suitable organic bonding agents. The bondingagents may typically be phenol-formaldehyde or epoxy resins or otherthermosetting-type resins with which the mass of typically interrelatednonwoven glass fibers are impregnated during the formation process ofthe fibers into their particular form or thereafter. However, thechemical and temperature resistance of the filter so prepared ismodified by the employment of such bonding agents. Typically an epoxyresin is used which provides good strength and fairly high-temperatureand chemical resistance to a self-supporting nonwoven tubular fiberfilter. Such filter tubes are described more particularly in U.S. Pat.3,767,054, issued Oct. 23, 1973. Although such epoxy resin glass fiberfilter tubes are suitable for many uses, the organic bonding agentrestricts the scope of the filter applications available, particularlysince the maximum temperature of use of such tubes is not over 200°C. Inaddition, such filter tubes are not very resistant to degradation in usewith many liquids, such as concentrated acids. Furthermore, such filtertubes, due to the presence of the organic resinous bonding agent, oftenhave an offwhite-to-light-brown color which darkens with age andsunlight due to the presence of such bonding agent. Thus, such filtersmay not be employed where a color indicator is employed with the filter,or where a white color is desirable.

SUMMARY OF THE INVENTION

My invention concerns filters and the method of preparing such filtersand using such filters. In particular, my invention relates to animproved nonwoven fibrous filter tube composed of inorganic fibers,particularly glass fibers with a hardened silicone resin as a bondingagent. My improved filters may be used in steam filtration applications,have improved resistance to acids, and are characterized by a lightwhite color and high hydrophobicity.

My invention provides a filter tube composed of glass fibers, whichfilter tube is similar or better in filtration properties; e.g., flowrate and efficiency, as prior art filters with epoxy resin as a bondingagent, but of different physical properties which permit my filter tubesto be employed in demanding uses not possible by such epoxy resin tubes.My filters, which are composed of inorganic fibers interrelated in anonwoven, randomly disposed pattern defining interstices of desiredporosity therebetween, and bonded together into a self-supportingcoherent mass or form, such as a tube, with hardened silicone resinbinder, have improved temperature resistance, good color, improvedresistance to concentrated acids, and are more hydrophobic than epoxyresinbonded filters.

My invention provides for a mat, or mass in desired form, ofinterplaced, overlapping or interrelated inorganic fibers having variousdiameters; for example, having a diameter from about 0.001 to about 10microns; for example, 0.03 to 8 microns, such as 0.1 to 5.0 microns,bonded together by a hardened cross-linked silicone resin binder.

A wide variety of fibers, but particularly inorganic fibers, may beemployed in preparing the self-supporting filters of my invention.However, particular types of inorganic fibers that are suitable for thefibers in my invention are, for example, preferably glass fibers,particularly borosilicate glass fibers, alumina fibers, zirconia fibersand the like and combinations thereof.

The treatment of the fibers with the silicone resin bonding agent,particularly where the hardenable resin is applied to the fiber mass asa liquid or silicone resin solution, such as by immersion, also coststhe fiber surface, and with the resin at the crossover junctions,imparts hydrophobicity to the filter. The filters are bonded togethergenerally at junctions wherein the fibers cross or contact each other inthe particular form by the hardened or rigid silicone resin agent.

The term "hardened silicone resin" is employed to define a silicone-typeresin material containing a plurality of SiO groups as a backbone of thepolymer chain, such as a silicone resin prepolymer prepared by thecohydrolyzing of organochlorosilanes and mixtures, to form a resin highin silanol groups (=SiOH). The resin is partially condensed to form apolysiloxane prepolymer with a few silanol groups. The solvent solubleresin, on the application of heat and/or a catalyst, or preferably both,is then condensed further, polymerized or reacted to form a cured orcrosslinked thermoset polysiloxane resin. Typically, the by-product ofthe condensation reaction is water. The silicone resin is composed thenof cross-linked Si-O- bonds. Other suitable hardenable silicone resinsinclude silicone elastomers which are either one or two-componentsystems curable at room temperatures, or at slightly elevatedtemperatures, and flexible silicone resins typically derived fromdimethylpolysiloxane. The rigid elastomers and flexible resins aredescribed more particularly in "Modern Plastics Encyclopedia", Vol. 50No. 10A, October 1973, pages 102 and 107, hereby incorporated byreference.

Such hardenable silicone resins are well-known and commerciallyavailable. The silicone resin is typically supplied in liquid orsolvent-containing form and is hardened into a semirigid or rigid resinwhich normally is solvent-resistant, thermosetting, white in color andhydrophobic in properties. A wide variety of silicone resins may beemployed as bonding agents, such as those silicone resins which areelastomers, coating resins, or foamable resins.

My filter may be treated with the liquid resin, or more preferably andeasier with the silicone resin, in a suitable volatile organic solvent.The solution may contain typically 2 to 50%; e.g., 5 to 20%, by weightof the silicone resin. The solvents selected for the solution should bethose solvents which may be removed by evaporation, such aslow-boiling-point organic liquids; e.g., less than 100°C. Preferredsolvents include, but are not limited to, low-boiling organic ketoneslike acetone, and methyl ethyl ketone, and halocarbons likehalohydrocarbons such as 1,1,1-trichloroethane, carbon tetrachloride,and fluoro and chloro derivatives of methane, ethane and propane; e.g.,the Freons (a trademark of DuPont de Nemours & Co.), or combinationsthereof. Suitable diluents, such as hydrocarbons, alcohols, esters,ethers, etc., may also be used.

The silicone resins are crosslinked into a rigid three-dimensionalstructure by the use of elevated temperatures; for example, over 100°C,and typically 150° to 250°C, for times of 5 minutes to several hours;e.g., 1/2 to 2 hours. Various additives may be employed as desired withthe silicone resin, but normally a catalyst or combination of catalystsor compounds are employed in micro amounts to speed up the cure time,the hardness of the resin, or to reduce the cure temperature or increaseresin heat life. Such catalysts are well-known and include variousorganometallic compounds; for example, metals of zinc, cobalt,manganese, and tin, with oil-soluble fatty acids and naphthenatic anionsas well as various amine compounds. Some such catalysts include, but arenot limited to, zinc octoate, zinc naphthenate, cobalt and manganeseoctoates and naphthenates, amines such as choline octoate, dibutyl tindilaurate, etc. The catalysts are employed in very low amounts; forexample, 0.01 to about 2.0% by weight of the silicone resin, such as 0.1to 1.0% by weight of the metal on the silicone resin solids.

My invention is also directed to a method of preparing my improvedfilters and the use of such filters in filtering processes. The methodof preparing such filters comprises dispersing the particular fibers orcombinations thereof in an aqueous solution; forming the fibers into aparticularly desired, randomly disposed configuration to forminterstices of desired porosity, the fibers typically in flat sheet orpreferably in tubular form to form a mat of interrelated nonwovenfibers; and drying the fibers so formed. In one embodiment, my methodcomprises dispersing inorganic fibers, such as borosilicate glassfibers, in an aqueous solution; forming a flat sheet or tubular mat ofnonwoven interrelated fibers; impregnating the matted fibers with thehardenable silicone resin bonding agent; and drying the impregnatedmatted fibers to provide the improved coherent unitary filters of myinvention.

The filters of my invention are prepared in the same manner as epoxyresin filters. For example, glass fibers are dispersed in an aqueoussolution. In the preparation of a tubular filter, a wet matted tube ofglass fibers is formed onto a tubular porous mandrel by immersing themandrel into the glass fiber dispersion, and, for example, removingwater by draining or preferably with the aid of a vacuum attached to theinterior of the mandrel. Typically, the mandrel may be of stainlesssteel mesh having holes covered with a woven stainless steel wire.

In one embodiment, the filber tubes so prepared are integral, unitary,self-supporting tubes having crushable ends to form, by the axialcompression of the fibers at or near the ends, a peripheral seal so thatsealing gaskets need not be employed.

In the method of producing my filters, another watersoluble binder maybe introduced in small amounts into the aqueous dispersion of theinorganic fibers, such as a silica sol, so that the fibers may be keyedtogether prior to the further stage of the bonding with the siliconeresin bonding agent. Preferably the fibers are formed without the use ofa bonding agent, and the nonwoven fibers after drying are subsequentlyimpregnated with the desired silicone resin solution.

In general, a solution of silicone resin is employed to provide forabout 3 to 50%; e.g., 5 to 40%, by weight of the resin of the filter.The glass fiber dispersion containing the organic binder is maintainedto keep the glass fibers in optimum condition for the mutual bonding ofthe glass fibers to one another.

Once the inorganic fiber mat has been formed as described, the materialis then dried in a steam oven or similar drying apparatus to provide adried filter tube. Impregnation of the binder material onto the nonwovenmass of fibers is then accomplished by treating the dried filter withthe silicone resin binder. For example, the dried filter is impregnatedwith the liquid silicone resin by immersing, spraying, coating, orotherwise treating the tube with a solution, dispersion, emulsion, orbulk liquid resin.

In the preferred embodiment, the dried filter tube is immersed in avolatile organic solvent-containing silicone resin solution. The tube isthen gently heated to remove the solvent and the tube then heated to atemperature, for example, 100° to 200°C, and for a time sufficient tocross-link and cure the silicone resin.

The filters prepared in accordance with my invention may, of course,take any form; for example, in the form of flat sheets, discs or tubes.In the case of tubes of glass fibers in particular, such tubes areself-supporting and self-gasketing; i.e., the fibers at the end of thetube are compressible to form a seal, and often no gasketing or sealingis necessary to connect the filter tubes to the manifold. If desired,the filter tubes may be used alone or may be used in conjunction with aporous support sheet or filter core, or a filter assembly comprising anexternal housing, with means to admit a fluid into or withdraw a fluidfrom the housing and the interior of the tube.

My invention will be described for the purposes of illustration only,and in particular in connection with the preparation of aself-supporting semirigid borosilicate glass fiber silicone resin-bondedtube.

DESCRIPTION OF THE EMBODIMENTS

A filter tube is prepared by adding a volume of water to a beater, andwhile circulating the water, adding a quantity of the borosilicate glassmicrofibers to disperse the fibers. Filter tubes are formed byintroducing a vacuum-type mandrel into the furnish to form a wet mass ofnonwoven microfibers on the mandrel surface of the desired thickness.After formation, the tubes are then racked and dried at a temperature offrom about 70 to 90°C. The dried tube is then immersed in a siliconeresin solution composed of approximately 7% by weight of a curablepolysiloxane resin (I.C.I. resin R 282 resin) with 0.5% zinc catalyst asa curing agent in a solution of methyl ethyl ketone. The immersed tubeis removed, the solvent evaporated, and then the tube is cured at 200°Cfor about one hour. The dried tubes are then cut to the desired lengthfor use. The resulting dried filter tube of my invention is composed ofapproximately 75% borosilicate glass microfibers and 25% silicone resinbinder.

My tubes so prepared can be used for steam filtration applications,since they have good chemical resistance to steam. Steam of up to 100psig (168°C) can be filtered for thirty 10-minute cycles during whichflow is reduced from full (to atmosphere) to minimum, with only minorstrength reduction (25%).

A comparison of the steam resistance of my tubes and standard epoxyresin tubes prepared in the same manner showed that after a steam cycleof 20 psi for 20 minutes, the strength of standard tubes was 30% oforiginal, while my silicone tubes were 90% of original.

The superior resistance to steam makes my tubes superior for sterile airfilters which must be sterilized by steam. In addition, the markedlyincreased hydrophobicity of my filter gives reduced condensate retentionand higher flow capacities post steaming.

The reduced condensate retention of my tubes was demonstrated bysteaming a standard tube and my tube of the same grade for 20 minutes at20 psig. Immediately after steaming, air was passed through each tube at80 l.p.m. and the pressure differential across the filter plottedagainst time. The pressure differential across my tube was increased 67%by condensate retention and then fell steadily to 6.7% after 30 minutes.By comparison, the pressure differential across a standard tube wasinitially increased 217% and then fell to 83% after 30 minutes.

My silicone resin tubes show a marked increase in their resistance todegradation by strong acids, and, therefore, can be used for thefiltration of such chemicals.

    ______________________________________                                                Initial                                                                              Collapse     Collapse                                                  collapse                                                                             strength after                                                                             strength after                                            strength                                                                             72 hr. soak in                                                                             72 hr. soak                                               (psi)  10% HCl. (psi)                                                                             in 50% HNO.sub.3                                                              (psi)                                             ______________________________________                                        Standard tube                                                                           29.5     19.5         9.5                                           My tube of                                                                              27       32           29                                            Example                                                                       ______________________________________                                    

The silicone resin is white in color and produces white tubes which arecolor-stable, unlike standard tubes. Standard tubes vary from off-whiteto brown or green on long exposure to sunlight. This color instabilityof standard tubes makes them unsuitable for use as outer porous sleevesin composite filter elements which incorporate a color indicatormechanism; e.g., an absorbent activated carbon-filled cartridgeincorporating an organic soluble dye to indicate the presence of liquidoil. When the dyed oil appears at the outer filter sleeve, its color canbe masked by undesired discoloration.

The extreme hydrophobicity of my tubes finds useful application incertain areas of filtration, such as coalescing or phase separation ofemulsions, such as oil-in-water emulsions or removing oil droplets froma moisture-laden gas stream, and in other applications where hydrophobicfilters are desirable. Further, my filter tubes may be used totemperatures of 250°C, while standard tubes are not suitable over 200°C.

My invention has been described in reference to the preferredembodiment; however, as will be apparent to a person skilled in the art,various modifications, changes and additions may be made withoutdeparting from the spirit and scope of my invention as described.

What I claim is:
 1. A filter tube, which tube comprises a plurality ofinterrelated, randomly disposed inorganic glass fibers havinginterstices therebetween to define the porosity of the filter, thefibers having a diameter ranging from about 0.001 to about 10 micronsand bonded at the junctions of the fiber crossover points with a bondingagent consisting essentially of a hardened silicone resin, the fibersinterrelated to form a semirigid porous self-supporting fibrous filtertube.
 2. The filter of claim 1 wherein the fibers are composed ofborosilicate glass fibers.
 3. The filter of claim 1 wherein the fibershave a diameter ranging from about 0.03 to about 8 microns.
 4. Thefilter of claim 1 wherein the filter is in the form of a nonwovensemirigid self-supporting tube, the fibers at the ends of the tubeadapted to be compressed together in a sealing relationship on theapplication of an axial force.
 5. The filter of claim 1 wherein thebonding agent comprises from about 3 to 40% by weight of the filter. 6.The filter of claim 1 wherein the hardened silicone resin comprises arigid cross-linked polysiloxane resin.
 7. The filter tube of claim 1which includes an outer porous filter sleeve, the filter sleevecontaining a soluble dye therein to indicate the presence of liquid oil.8. A filter tube, which tube comprises in combination: a tubular mass ofinterrelated, randomly disposed nonwoven glass fibers, the fibers havinga diameter of from about 0.03 to 8 microns, the glass fibers bonded atthe junctions of the fiber crossovers in the filter with a hardenedcross-linked polysiloxane resin bonding agent in an amount ranging fromabout 3 to 50% by weight based on the weight of the filter, the glassfibers interrelated to form a semirigid self-supporting filter tube ofthe desired filtering porosity.
 9. The filter tube of claim 8 whichincludes an outer porous filter sleeve composed of activated carbonwhich incorporates an organic soluble dye therein to indicate thepresence of liquid oil.
 10. A method of filtering a steam stream at ahigh temperature of up to about 168°C with reduced filter tubecondensate retention, which method comprises: passing the stream ofsteam which is desired to be filtered at a temperature of up to about168°C and a pressure of up to 100 psig through a filter materialcomprising a plurality of interrelated glass fibers, the fibers bondedat the junctions of their fiber crossovers with a hardened siliconeresin bonding agent, the fibers interrelated to form a semirigid porousmass of desired filtering porosity.
 11. The method of claim 10 whereinthe filter is composed of borosilicate glass fibers having a diameter offrom about 0.001 to about 10 microns, the borosilicate glass fibersbonded together by a rigid cross-linked silicone resin bonding agent.